Unpolarized light waves vibrate in a large number of planes about the axis of a light beam. If the waves vibrate in one plane only, the light is said to be plane polarized. Several useful optical systems can be implemented using polarized light. For example, electro-optical devices such as liquid crystal display screens are illuminated with polarized light and use crossed polarizers in conjunction with an addressable liquid crystal interlayer to provide the basis for displaying image formation. In the field of photography, polarizing filters have been used to reduce the glare and the brightness of specular reflection. Polarizing filters, circular polarizers or other optical components have also been used for glare reduction in display device screens.
Several different kinds of polarizing film are available for polarizing unpolarized light. Absorbing (dichroic) polarizers have, as an inclusion phase, polarization-dependent absorbing species, often an iodine-containing chain, that are aligned within a polymer matrix. Such a film absorbs light polarized with its electric field vector aligned parallel to absorbing species and transmits light polarized perpendicular to the absorbing species. The optical properties of such a film are typically specular, with very little diffuse transmission through the film or diffuse reflection from the film surfaces.
Another type of polarizing film is a reflective polarizer, which separates light in different polarization states by transmitting light in one state and reflecting light in the other state. One type of reflective polarizer is a multilayer optical film (MOF), which is a film formed of a stack of many layers of alternating polymer materials. One of the materials is optically isotropic while the other is birefringent, with one of its refractive indices matched to that of the isotropic material. The layer thicknesses may be varied throughout the stack so as to be quarter wave layers over a wide range of wavelengths, for example over the visible region. Light incident in one polarization state experiences the matched refractive indices and is substantially specularly transmitted through the polarizer. Light incident in the other polarization state, however, experiences multiple coherent or incoherent reflections at the interfaces between the different layers and is reflected by the polarizer. Since the alternating polymer layers are substantially planar, the reflected light is mostly specularly reflected.
Another type of reflective polarizing film is constructed from inclusions dispersed within a continuous phase matrix. The inclusions are small relative to the width and height of the film. The characteristics of these inclusions can be manipulated to provide a range of reflective and transmissive properties to the film. The inclusions constitute a disperse polymer phase within the continuous phase matrix. The inclusion size and alignment can be altered by stretching the film. Either the continuous phase or the disperse phase is birefringent, with one of the refractive indices of the birefringent material matching to the refractive index of the other phase, which is optically isotropic. Selection of the materials for the continuous and disperse phases, along with the degree of stretching, can affect the degree of birefringent refractive index mismatch between the disperse phase and the continuous phase. Other characteristics that can be adjusted include the inclusion size with respect to wavelength within the film, the inclusion shape and the inclusion volumetric fill factor. In such systems, light polarized to experience the refractive index mismatch between the disperse and continuous phases is diffusely reflected, whereas the orthogonally polarized light is specularly transmitted.